BBFC (British Board of Film Classification) is a company that classifies the certificate rating of films in britian, they rate all films and say what certificate they should be .
The BBFC rates films into the following catorgories: U PG 12 12A 15 18 R18.

15:
15 rated certificate majoritively allow moderate violence, gore and sexual activity, but not too frequent and must be relevant. Drug taking is not banned but should not be promoted.

18:
18 rated certificates allow gore, violence, sexual activity and drug taking. again, not to be promoted. The difference between the two certificates is diciphered by it's explicity.

Details found from: http://www.bbfc.co.uk/